When a drive transistor, which is connected to a load so as to drive the load (for example, a solenoid), is turned on and off for pulse width modulation (PWM) control such that the load is driven, electromagnetic inference (EMI) noise occurs due to high-frequency components of an output voltage from the drive transistor.
For this reason, EMI noise is reduced by sufficiently ensuring an ascending time and a descending time (hereinafter, also referred to as a gradient or a slope) for the waveform of a voltage driving a load via the charging or the discharging of the capacitance of an ON/OFF control terminal (for example, a gate terminal of a drive MOS transistor) of the drive transistor for driving the load with a current source (For example, PTL 1). Typically, a relationship between a voltage waveform and a frequency spectrum is used as electromagnetic compatibility (EMC) technology (For example, NPL 1).
With respect to a voltage waveform in which trapezoidal waves with a period T, a pulse width Pw, an ascending time tr, a descending time tf, and an amplitude A are repeated as illustrated in FIG. 4 (a), when tr is equal to tf, as illustrated in FIG. 4(b), a high-frequency spectrum with a fundamental frequency of 1/T forms an envelope curve.
In FIG. 4(b), since the number of high-frequency components is small in a frequency region from a low frequency to a frequency of f1=1/(πPw), a frequency characteristic is approximately flat, and in a frequency region from a frequency of f1=1/(πPw) to a frequency of f2=1/(πtr), high-frequency components decrease at a ratio of −20 dB/decade when the frequency increases. In a frequency region from a frequency that is higher than or equal to f2=1/(πtr), high-frequency components decrease at a ratio of −40 dB/decade when the frequency increases.
From the relationship between the voltage waveform and the frequency spectrum, it is understood that an increase in the ascending time and the descending time for a signal waveform is effective in reducing the frequency spectrum in the high-frequency region. When tr is not equal to tf, f2 is mainly determined by small values of the ascending time tr and the descending time tf.